Hydrocarbon adsorber systems for preventing escape of hydrocarbon vapors into the atmosphere are well known. Vehicles are routinely provided with canisters containing activated carbon for trapping fuel tank vapors displaced when a fuel tank is refilled. To meet very low emissions standards, the outlet of a canister may be further provided with a high-efficiency adsorptive carbon monolith scrubber for scavenging traces of vapor which may pass through the main canister. After the fuel tank is filled and the engine restarted, a connection of the adsorber system to vacuum in the engine's intake manifold causes atmospheric air to be drawn through the scrubber and canister, gradually desorbing the adsorbed hydrocarbons and presenting them to the intake manifold for engine combustion.
A post-canister scrubber is typically a high-efficiency device having low capacity. Therefore, it is very important that the scrubber be completely purged of adsorbed hydrocarbons during each desorption cycle. In the prior art, there is not a means for determining the hydrocarbon level in an activated carbon device, nor for determining when desorption is complete. One approach would be to install a hydrocarbon sensor adjacent to the scrubber to measure the hydrocarbon content of purge air passing out of the scrubber and into the canister. This approach would add significantly to the cost of a vehicle.
For some vehicles, as for example Hybrid vehicles, in order to meet PZEV standards for vehicle emissions, heating of a carbon scrubber and canister to allow efficient purging thereof is required. A carbon scrubber acts as a resistor when voltage is applied across it, causing the scrubber to heat up and thereby accelerating the purging of adsorbed hydrocarbons.
A disadvantage of electric heating of an adsorptive device is that it is wasteful of electricity, which makes it a significant power parasitic.
An additional disadvantage is that if the electric current is maintained after desorption of the scrubber and canister is complete, the temperature of the carbon in both adsorbers will continue to rise, thereby progressively reducing the desorption capacity of the hydrocarbon adsorption system while wasting additional electricity. Therefore, for both these reasons, it is desirable to shorten the period of desorptive heating to the minimum time required for completion.
What is needed in the art is method and apparatus for determining simply and reliably when desorption of an adsorber is complete, in order to be able to shut down the scrubber's heater at the minimum required heating period.
It is a principal object of the present invention to minimize the total electricity required for a desorption cycle of an electrically heated hydrocarbon canister and scrubber system.
It is a further object of the invention to shut down the heater at the earliest possible time, thereby presenting the coolest possible canister and scrubber for the next cycle of adsorption.